Cell Membrane 1

Also called plasma membrane of a cell. Primarily the cell membrane is made of phospholipids and associated proteins. Cell membrane is a double layer of molecules so is also called lipid bilayer. The phospholipids have a polar end and a fatty acid end. So its polar end is hydrophillic and the fatty acid end is hydrophobic.

The cell membrane seperates extracellular aqueous environment from the intracellular aqueous environment. In the lipid bilayer the fatty acid ends of phospholipid are facing each other and they make the central part of the cell membrane. Now as the core of the cell membrane is made of lipid thus only lipid soluble substances can pass through the cell membrane or substances with very small molecular weight can pass through diffusion. Polar molecules cannot pass through the phospholipid bilayer, thus there are special type of proteins present in the membrane that allow these polar molecules to pass. These protein molecules are made of
peptide chains and these peptide chains are arranged in such a manner that there is a longitudinal tunnel which is full of water known as aqueous channel that allows these polar substances to pass through. These proteins are called Channels. The phospholipids present in the outer leaflet of the membrane are slightly different from the phospholipids present in the inner leaflet. 

Phospholipids that are present in the outer leaflet are Phosphatidylcholine and Sphyngomylin. Phospholipids that are present in the inner leaflet are Phosphatidyl ethanol and Phosphatidyl serine. Thus the outer and inner leaflets of the membrane show some asymetry. Also proteins attached to the outer and inner leaflet are slightly different. Moreover the outer leaflet has carbohydrates attached to it whereas the inner leaflet has none. 

Cholestrol is a molecule that is present in the cell membrane intercalated between the nonpolar or fatty acid ends of the phospholipids. The cholestrol molecules have hydroxyl ends oriented outwards because these are polar. These phoispholipid molecules are mobile, they keep moving laterally. This is called Fluidity.

Fluidity is an important property. Why? lets understand with examples:

1. Lets talk about the cells in pancreas. Pancreas secrete insulin. In order to do that insulin containing vesicles fuse with the cell membrane and a puncture point is formed from where the insulin molecules are released. Once molecules are released the fluidity of allows the membrane to seal off this puncture and the cell remains intact. If the membrane were fixed this fusion of the vesicle and secretion of the insulin would not be possible niether would it be possible for the cell to seal the puncture. Thus fuildity allows Exocytosis.
2. Another example is Endocytosis. In this a substance enters the cells by invagination of the membrane followed by a piece of membrane breaking off and surrounding the molecule and in this way the molecule enters the cell. the Hole where the part of the membrane broke off is resealed, thanks to the fluidity of cell membrane.
3. This fluidity is also important for Membrane Biogenesis (growth). Cells synthesize new phospholipids that are inserted in the membrane and as the membrane is fuild thus the preexisting phospholipids move and make place for the new ones.

What are the factors that alter the fluidity of the membrane?

1. Temperature: Increased temperature increases the membrane fluidity because it increases the movement of the molecules and vice versa.
2. Cholestrol: Increased colestrol decreases the fluidity of the membrane because the components are more tightly packed. In other words the fatty acid ends are saturated.

In some parts of the cell membrane phospholipids are more tightly packced and there fatty acid components are longer and the cholestrol are also packed tightly in between. This is a specialized area in the membrane. In this area are present specialized proteins that are receptor preteins. 

What is a receptor?
Receptors are special macromolecules that can bind to a specific substance (hormone, neurotransmitter, drugs) and give a signal to the biological system.

Some substances are lipid soluble and thus their receptors are present within the cells. Example: thyroxine.
Some subsatances are polar and thus there receptors are present on the membrane surface . Example: insulin.
So the receptors and their associated proteins in the specialized area of the cell membrane are called Lipid Rafts.